exactly. One makes the head, two make each arm, etc...

of course, you can't do that with computer software

You either never read the book or are interpreting it wrong.

The premise is that as you add more people to an EXISTING project, the amount of time it will take to complete will increase. However, if all of those people are involved from the beginning, then you are not subject to the same phenomenon - within reason of course.

So while it might not be "6" developers working on it for a year, it could potentially be 9 very devoted developers cranking this out with good results towards the end of the year.

Errmm, if you actually read the book properly you'll realise that the baby/mothers analogy applies to synchronous sets of work and workloads that cannot be broken down into parallel running tasks. Once architected to a suitable degree I'd imagine that there are at least a few streams of work that could be carried out concurrently.